The present invention relates to a method for growing a semiconductor crystal from the vapor phase by adding or doping a metal element into a semiconductor substrate by gas decomposition process.
When a metal impurity is doped into a semiconductor so as to lower its energy level, the so-called "semi-insulating" semiconductor crystal having a suppressed conductivity may be obtained. For instance semi-insulating crystals added or doped with Cr find a variety of applications; that is, as a buffer layer in a field-effect transistor, an embedded layer in a semiconductor laser and so on. However there had not been proposed a method for permitting the addition or doping of a metal impurity such as Cr into a GaAs crystal in a simple manner. The existing apparatus are all extremely large and complex in construction yet incapable of providing crystals with desired qualities. For instance, when a semi-insulating GaAs film is epitaxially grown, a chloride of Cr, Fe or Ni must be heated to a high temperature so that a metal impurity may be doped into the GaAs film under high vapor pressure. More particularly in the process for epitaxially growing a semi-insulating GaAs film by the conventinal disproportionation, metallic gallium placed in a first zone in a reaction tube is made to come into contact with a gas mixture consisting of HCl and H.sub.2 so that gaseous GaCl may be charged into a second zone wherein GaCl reacts with AsH.sub.3 to yield GaAs. Concurrently FeCl.sub.2 which is charged into the second zone through a separate feed line is decomposed into Fe which in turn is doped into GaAs to be deposited on a GaAs substrate placed in a third zone in the reaction tube. However the vapor pressure of FeCl.sub.2 is low so that the vapor pressure must be increased by heating in order to permit Fe to be doped into GaAs. For instance, FeCl.sub.2 must be heated to 700.degree. C. so that it may have a vapor pressure of 10 Torr. To this end a preheating means must be provided so that not only a source of FeCl.sub.2 but also a line for charging heated FeCl.sub.2 into the furnace tube may be maintained at 700.degree. C.
Furthermore when a field-effect transistor is fabricated, a semi-insulating GaAs film or a buffer layer is first grown on a substrate and then a GaAs film having a low impurity concentration or an active layer is grown on the buffer layer. However the charge of heated FeCl.sub.2 into the reaction tube cannot be instantaneously suspended in practice because of the difficulty of closing a stop valve inserted in the FeCl.sub.2 supply system which is heated to high temperatures. As a result the doping of Fe into the active layer cannot be avoided at the early stage in the crystal growth of this active layer so that the sharp change in resistivity cannot be attained at the interface between the buffer and active layers. Consequently the dimensional control of a channel region in a field-effect transistor is difficult and desired characteristics cannot be obtained.